1. Field of the Invention
The invention concerns an apparatus for supplying direct current from a three-phase alternating-current source.
2. Description of the Prior Art
Rectifying apparatus for the supply of direct current (D.C.) consumers or loads generally consists of a mains tranformer or transformers, a rectifier unit and adjusting, regulating or control elements in the primary or secondary circuits.
The construction of these main units is rendered difficult by the disadvantageous effect of certain factors relating to their material and floor space requirements. The characteristic performance (rated output) of the transformer is determined not only by the circuit arrangements of the included rectifier and the rated (nominal) D.C. output, but also by the maximum D.C. output voltage required.
If the desired V-I characteristic of the rectifying devices has a flat and/or falling shape, i.e. a reducing D.C. output current is associated with a substantially higher D.C. output voltage than the D.C. voltage associated with the rated D.C. output, then the rated performance of the mains transformer of the rectifier equipment (i.e. the characteristic factor on which the material and floor space requirement of the transformer chiefly depends) is determined by a maximum D.C. output voltage at which the rated output of a transformer is consequently considerably higher than that of a transformer determined by the D.C. output voltage associated with the nominal D.C. current. Such a transformer therefore requires more floor space.
In some practical applications the need for an increase in the rated performance of the mains transformer has been obviated by the inclusion into the equipment of certain auxiliary supply units that provides the higher D.C. output voltage that goes with the lower D.C. output current.
This solution has led, however, to dividing the rated output of the equipment between two supply units, resulting, therefore, in no tangible advantage regarding material and floor space requirements.
One known way to overcome these difficulties is by the use of D.C. regulators. These are, however, rather complicated devices and their application is therefore limited.
Owing to their known practical advantages, rectifier units are often used in the form of a six-pulse three-phase half-wave circuit. Such rectifiers are fed by secondary coils connected to individual legs of an alternating current transformer. To achieve an economic operation of the semiconductors and to ensure longer current conduction times, and also the parallel current conduction of the semiconductors, saturable reactors are often built into the equipment. The regulating elements in the primary or secondary circuit are based on a magnetic working principle. Also, Triacs are widely used in the primary circuit, and so are controlled rectifying elements such as thyristors, namely in the secondary circuit.
The disadvantage of regulating elements being included in the primary and secondary circuits such as magnetic amplifiers is that they again require a large floor space.